PV systems are becoming increasingly popular for power generation in part because of low maintenance cost, and an example of conventional PV generators 100-A and 100-B can be seen in FIGS. 1 and 2. In these examples, the PV elements 102-1 to 102-N are coupled together in a string (i.e., in series with one another) in a differential network 110-A and 110-B. The PV elements 102-1 to 102-N can be individual cells, strings of cells, panels (which would include multiple cells), strings of panels, and so forth. The differential network 110-A and 110-B can be employed with the PV generators 100A, 100B so as perform balancing (e.g., maximum power point tracking or MPPT) among the elements (e.g., 102-1 to 102-N) with differential converters. As shown, there are PV elements 102-1 to 102-N arranged in a sequence such that the first PV element (i.e., 102-1) and last PV element (i.e., 102-N) are coupled to central converter 106 (which can, for example, be a direct-current-to-alternating-current (AC/DC) converter or a direct-current-to-direct-current (DC/DC) converter). The differences between generators 100-1 and 100-B, though, lies in the topology of the DC/DC converters.
Turning first to FIG. 1, the differential converters are arranged as DC/DC converters. As such, there are intermediate nodes ND-1 to ND-(N−1) between PV elements 102-1 to 102-N, and coupled to each intermediate node ND-1 to ND-(N−1) is an inductor L-1 to L-(N−1). Each of these inductors L-1 to L-(N−1) is also coupled to a switching node SW-1 to SW-(N−1), which, as shown in this example, are between switch pairs S-1,1/S-1,2 to S-(N−1),1/S-(N−1),2. The inductors L-1 to L-(N−1) and switch pairs SA-1,1/SA-1,2 to SA-(N−1),1/SA-(N−1),2 together with the corresponding DC/DC controllers 104-1 to 104-(N−1) form the DC/DC converters (which can, for example, be buck converters, boost converters, or buck-boost converters) that allow for balancing.
Alternatively, the differential converters can be arranged as flyback converters as shown in FIG. 2, or forward converters (not shown). In the example configuration of FIG. 2, the primary sides of transformers TR-1 to TR-N are respectively coupled across PV elements 102-1 to 102-N, with conduction across the primary sides of transformers TR-1 to TR-N being controlled by switches SB-1,2 to SB-N,2, respectively. The secondary sides of each of transformers TR-1 to TR-N are coupled across the central converter 106 with conduction across the secondary sides of transformers TR-1 to TR-N being controlled by switches SB-1,1 to SB-N,1. The switches SB-1,1 to SB-N,1 and SB-1,2 to SB-N,2 are controlled by flyback controllers 108-1 to 108-N in local controller 108-B.
Balancing, while useful, may prove to be insufficient. Utility scale power generation stations may include hundreds of thousands of PV elements (e.g., 102-1 to 102-N), and, in order to obtain close to optimal performance, close monitoring of the elements (e.g., 102-1 to 102-N) may be desirable. Shade (e.g., from dust) or damage (e.g., from hail) may cause one or more of the elements (e.g., 102-1 to 102-N) to operate at a less than desirable level. Manually checking each individual element (e.g., 102-1 to 102-N) is costly in terms of labor and other costs, so it is desirable to be able to automatically monitor the elements (e.g., 102-1 to 102-N). Monitoring, though, typically implies measurement of the PV current for each of the elements (e.g., 102-1 to 102-N), and this monitoring has transitionally been accomplished with invasive sensors (e.g., sense resistors inserted into the strings), which can be costly, increase energy losses, and be difficult to manage by themselves.
Therefore, there is a need for a method and/or apparatus for monitoring PV elements.
Some examples of conventional systems are: U.S. Patent Pre-Grant Publ. No. 2011/0115297; U.S. Patent Pre-Grant Publ. No. 2012/0098344; U.S. Patent Pre-Grant Publ. No. 2011/0031816; U.S. Patent Pre-Grant Publ. No. 2012/0091800; and Shenoy et al., “Differential power processing architecture for increased energy production and reliability of photovoltaic systems,” in Proc. IEEE Applied Power Electronics Conference and Exposition, pp. 1987-1994, 5-9 Feb. 2012.